The present invention relates to a method of reducing chlorates present in aqueous alkali metal hydroxide solutions. More particularly, this invention relates to a method of reducing chlorate impurities present in alkali metal hydroxide solutions by treatment with a hydroxycarboxylic acid.
Alkali metal hydroxide, e.g., sodium hydroxide or potassium hydroxide, is commonly produced by the electrolysis of aqueous alkali metal halide solutions in an electrolytic cell, e.g., a diaphragm-type cell. A product, commonly known as cell liquor, is obtained from diaphragm-type cells. Cell liquor generally contains from about 10 to 12 weight percent alkali metal hydroxide, 12 to 16 weight percent alkali metal halide, minor impurities, e.g., alkali metal chlorate, and the balance water. Generally, the total amount of alkali metal chlorate impurities ranges from about 20 to 400 parts per million (ppm) on an anhydrous basis.
In the production of anhydrous alkali metal hydroxide from cell liquor, alkali metal chlorate is normally present therein as an impurity and must be completely removed prior to dehydration of the cell liquor at high temperatures. Otherwise, thermal decomposition of the alkali metal chlorate can produce nascent oxygen and result in severe corrosion of nickel evaporators and other equipment used in the process.
In the processing of alkali metal hydroxide from electrolytic cells, the cell liquor is passed customarily to an evaporator system wherein a portion of the water and most of the alkali metal halide are removed. Substantially all of the alkali metal halide will be removed by the time the alkali metal hydroxide concentration reaches about 50 weight percent. The 50 weight percent alkali metal hydroxide solution is further concentrated by heating under vacuum to attain alkali metal hydroxide concentrations of about 73 weight percent. Subsequent evaporation of the remaining water from 73 weight percent alkali metal hydroxide can produce anhydrous alkali metal hydroxide. At the temperatures utilized in the final evaporation stage chlorate impurities can result in corrosion of metal parts, particularly nickel parts. The reduction of chlorate levels in aqueous alkali metal hydroxide solutions, particularly electrolytic cell alkali metal hydroxide solutions, is necessary to avoid corrosion problems.
It is known that sucrose (U.S. Pat. No. 2,610,103) and sorbitol (U.S. Pat. No. 3,380,806) may be added to alkali metal hydroxide solutions to reduce the chlorate impurities. In contrast, U.S. Pat. No. 3,380,806 discloses that lactic acid (a hydroxycarboxylic acid) is ineffective in reducing chlorates. However, lactic acid has a boiling point of 103.degree. Celsius (C.) and may not be retained for a sufficient time to allow the reduction of the chlorate.